The present invention relates to a valve having damped opening, said valve comprising a valve body having an inlet port and an outlet port, a valve stem suitable for moving between a closed position in which said stem co-operates with a valve seat so as to isolate the inlet port and the outlet port from each other, and an open position in which the stem is spaced apart from the seat so as to put the inlet and the outlet ports into communication with each other, the valve further comprising an opening control chamber for the valve stem, which chamber is suitable for being fed with fluid so as to urge the valve stem to move towards its open position, a return spring for urging the valve stem to move towards its closed position, a piston suitable for sliding in the body in a closure movement for urging contact to be established between the valve stem and the seat, and hydraulic control means for causing the piston to move, which control means include a flow-rate limiter device for deferring the closure movement of the piston relative to the movement of the valve stem towards its open position.
The invention is particularly applicable to a valve used as a pressure limiter, i.e. a valve that, when the pressure at the inlet port reaches a determined limit pressure, opens so as to connect the inlet port to the outlet port, thereby limiting the pressure in the duct connected to the inlet port. In particular, such a pressure limiter is used in a hydraulic feed circuit of a hydraulic motor, by being connected to the feed and discharge main ducts of the motor in order to limit pressure in those ducts.
More precisely, the valve of the invention is, in particular, a pressure limiter valve that is used in the hydraulic feed circuit of a hydraulic motor that is designed to drive a mass presenting high inertia. In which case, there exists a higher risk of pressure increasing suddenly in the feed main duct while the motor is being switched on, and of pressure increasing suddenly in the discharge duct while the motor is being braked. Such increases in pressure are due to the inertia of the mass that is driven.
Conventional pressure limiter valves suffer from the drawback of giving rise to large jolts, due to the sudden increases in pressure. The response time of a valve is, in general, such that it opens only once the pressure threshold at the inlet port of the valve has already been exceeded. The valve opening then causes the pressure at the inlet port to fall suddenly to bring it to the limitation target pressure, and thus gives rises to an unpleasant jolt.
In order to remedy those drawbacks of conventional valves, DE 40 33 301 discloses a pressure limiter valve having damped opening of the above-mentioned type. That valve opens as from a first intermediate threshold that is lower than the nominal setting corresponding to the target pressure limitation, and then makes it possible for the pressure at the inlet port to increase gradually to the nominal pressure setting. More precisely, in that known valve, the valve stem can be spaced apart from its stationary seat in order to open the valve as soon as the pressure at the inlet port reaches an intermediate value corresponding to the initial setting of the return spring of that valve stem. The piston is disposed around the valve stem and is in abutment against the rear end of the spring, so that that piston, when it is moved, increases the return forces exerted by the spring on the valve stem, so as to urge said valve stem to return against its seat. The piston is caused to move by feeding fluid to a control chamber that is situated behind the spring and that is connected to the inlet port via the center of the valve stem, via a constriction. Moving the piston in the direction in which the spring is compressed is possible only subject to a damping chamber being emptied, that emptying being slow insofar as it takes place via constricted passages. That feature makes it possible to delay movement of the piston so that said piston moves after the valve has been opened due to the valve stem moving.
When the pressure increases at the inlet port, above the pressure setting of the spring, a plurality of cycles take place, during which the valve stem is alternately spaced apart from its seat by being moved in a first direction under the effect of the pressure and returned in the opposite direction so as to come back into abutment with its seat, under the effect of the combined return force from the piston and from the spring. Those cycles take place in succession until the piston comes into abutment against a step in the valve body, in which position the compression of the spring corresponds to the nominal setting of the pressure limiter valve, so that the valve normally remains open so long as the pressure at its inlet port is at least equal to said nominal pressure setting.
The valve of DE 40 33 301 suffers from the drawback of having a complex structure, in particular since, in order to cause the piston to move, which piston itself causes the return spring of the valve stem to be further compressed, it is necessary to form a plurality of passages of precise dimensions in the valve body, for feeding the control chamber and for emptying the damping chamber. Those passages are significantly long because the control and damping chambers of the piston are situated behind the spring, in positions remote from the inlet port, from which those chambers are fed.
EP 0 638 746 discloses a valve of the above-mentioned type, in which, in addition, the valve seat is formed on the piston, said piston being provided with a bore in which the valve stem is slidably disposed.
In EP 0 638 746, the fluid feed to the closure control chamber for causing the piston to move in the closure movement is limited by a constriction. That does indeed make it possible to defer the closure movement of the piston, but, insofar as that constriction also limits fluid flow out of that chamber, and insofar as such outflow is necessary for resetting the valve, that constriction slows down the resetting, so that the valve is not operational rapidly.
An object of the present invention is to improve that state of the art by proposing a valve having damped opening that is suitable, in particular, for being used as a pressure limiter, that has a structure that is simpler and more compact, and that is quicker to reset.
In the meaning of the present invention, the fact that opening is damped means that, before the valve opens fully, said valve is opened partially in a manner such as to control, over time, the head loss between the inlet and the outlet ports, without causing said head loss to decrease suddenly. In other words, the decrease in head loss is damped.
This object is achieved by the fact that the hydraulic control means for causing the piston to move include a damping chamber disposed in a manner such that fluid must flow out of said damping chamber in order to enable the piston to move in its closure movement, the fluid flowing out via the flow-rate limiter device that is disposed between the damping chamber and the outlet port.
In accordance with the invention, the valve seat is mounted to move. Thus, when the pressure in the opening control chamber for the valve stem reaches the setting of the return spring of said stem, said stem is, in a first stage, spaced apart from its seat so as to enable the valve to open. However, the hydraulic control means of the piston enable said piston to move in a manner such that the seat “catches up” the valve stem so as to close the valve again. If the pressure continues to increase in the opening control chamber for the valve stem, then said valve stem is spaced apart from its seat again, and then said seat “catches up” the valve stem again.
The invention thus re-uses the principle of the valve opening and closing in cycles until it is maximally open, so as to make opening gradual. However, unlike in DE 40 33 301, the seat of the valve stem is a moving part, and it is said seat that moves to catch up the valve stem and to close the valve again at the end of each cycle. Thus, the valve stem is not, in itself, caused to move in reciprocating motion so as alternately to open and to close the valve during each cycle, but rather it always moves in the same direction, and it is caught up by the seat which also moves in the same direction. Operation is thus simpler than in DE 40 33 301, thereby making it possible to limit the durations of the opening and closure cycles since it is not necessary, within each cycle, to reverse the movement of the valve stem. Thus, the damping is made more gradual than in DE 40 33 301. In addition, the piston in which the seat is formed for the valve stem does not act directly on the setting spring. Its hydraulic control means can also be extremely simple, and situated in the vicinity of the inlet port, without any complex machining being necessary.
In accordance with the invention, the closure movement of the piston is damped without detrimentally affecting resetting of the valve. After an operating cycle, the piston can return very rapidly to its rest position, unlike in EP 0 638 746 in which the return to such a position requires the fluid contained in the piston closure control chamber to flow out via a constriction.
In particular, the flow-rate limiter device serves to limit the flow of fluid in the direction in which said chamber is emptied, which emptying is necessary to the closure movement of the piston.
The flow-rate limiter device can be of any known type, and, in particular, a constriction, a nozzle, or a flow-rate regulator.
Advantageously, the opening control chamber for the valve seat is provided in the piston, and the hydraulic control means for causing the piston to move include a closure control chamber for the piston, which chamber communicates with said opening control chamber for the valve stem via the inside of the valve.
By means of these provisions, it is the same fluid source that feeds both the opening control chamber for the valve stem and also the closure control chamber for the piston. The feed cross-sections for those chambers, and the respective thrust cross-sections for fluid thrust against the valve stem and against the piston can be such that the valve stem moves more rapidly than said piston, in order to open the valve before the piston has been able to catch up said stem so as to close said valve again.
Advantageously, the opening control chamber for the valve stem communicates with the inlet port.
In which case, the valve having damped opening is, in particular, used as a pressure limiter.
Advantageously, the piston has at least one window disposed between the bore and a peripheral portion of the piston that is situated in a space communicating with the inlet port, and the valve stem has an axial bearing surface that is suitable for co-operating with a guide portion of the bore that is situated between the opening control chamber for the valve stem and said window, clearance provided between said axial bearing surface and said guide portion making it possible for communication to be established between said space and the opening control chamber for the valve stem.
Said space may be constituted by the closure control chamber for the piston. In a variant, the bore of the piston is stepped and has a smaller-diameter portion, defined by the valve stem and in which the opening control chamber for said stem is formed, and a larger-diameter portion remote from the valve stem and in which said closure control chamber for the piston is formed.